Which statement about page replacement algorithms is true?

• A. LRU uses recency; NRU uses reference bits; Second-Chance gives pages a second chance; CLOCK is an efficient implementation of Second-Chance; tradeoffs in overhead and accuracy
• B. LRU uses probabilistic sampling; NRU uses aging bits
• C. CLOCK is unrelated to Second-Chance
• D. NRU always outperforms LRU in all scenarios

Answer: (A)

When memory is full, the system must choose a page to evict, and different algorithms balance accuracy and overhead in different ways. LRU is driven by recency: it aims to evict the page that hasn’t been used for the longest time, which requires tracking usage history. Not Recently Used relies on a reference bit to mark whether a page has been touched recently; based on that, it groups pages for eviction in a cheaper, less precise way than full recency tracking. Second-Chance builds on this idea by letting a page that has been referenced a pass or two to stay in memory—its reference bit is cleared and the page is moved on, effectively giving it another chance before eviction. CLOCK implements this concept in a practical, efficient form: it uses a circular list with a single hand and a reference bit on each page, giving pages a second chance as the hand passes, and evicting only when it encounters a page with a cleared reference bit. Because these approaches trade exact LRU accuracy for lower overhead, CLOCK is considered an efficient approximation of Second-Chance, and NRU and LRU differ in how they track and use usage information. The other statements misstate these relationships (for example, LRU is not based on probabilistic sampling, CLOCK is related to Second-Chance, and NRU does not universally outperform LRU in all scenarios).